Muscle and bone bring animated characters to life

A system that automatically generates a biologically accurate skeleton and muscles could help movie and games animators create characters who move more realistically.

Computer animators working on video games or films such as The Incredibles usually start with a virtual wireframe model of a character's outer appearance. The wireframe moves using preprogrammed limb movements, which in turn allows the animator to work out how the character's outer layer should bend before overlaying skin and hair.

But in real life the way people look when they move is governed by their bones and muscles. By starting from the inside with a skeleton and working outwards by adding muscles it is possible to make very realistic animations that can even be used by surgeons (see Virtual humans aid surgery and product design).

However, although this bottom-up approach produces more realistic results, it can be very confusing for artists and is too slow for animators working on games or feature-length films, says Jian Zhang, a researcher at the National Centre for Computer Animation in Bournemouth, UK.

Outward appearance

Zhang and his colleague Xiaosong Yang have developed a system that takes the outward appearance of a character and automatically generates a skeleton and muscles to fit inside.

"It works by using geometric processing of the still character to determine where muscles should go," he says. "If there's an important muscle it will show through the skin so the tool can infer where the major muscles should be."

The software guesses at a suitable skeleton to attach the muscles to by adapting a standard humanoid skeleton to fit the character's overall shape. With skeleton, muscles and skin all in place, the character can be made to move in more realistic ways.

The software simulates how the muscles affect the outward appearance as they contract, relax and slide in relation to joints and other muscles.

Other mammals

"Our method could also be applied to characters based on other mammals," says Zhang. As long as the basic shape of an animal's skeleton is known and the skin is designed, it would work just as well, he says.

But the team first plans to improve the technique's accuracy in generating human-like characters. "We plan to add detailed knowledge of the human anatomy," he explains. "We hope to be able to take a character and fit a very accurate anatomy to it."

This could even have medical applications, says Zhang, allowing doctors to infer the biomechanics of a patient's anatomy from their outer appearance.

"That's a possibility if it is made more detailed," says George Leaver, a computer animation expert at Manchester University, UK. But he thinks animators will be using it first. "Because they've created a technique that integrates some anatomical modelling into what animators already do, this one might get taken up."

"A mistake people often make is trying to completely replace what animators already use with a new technique," he explains. "Having used these animation tools myself, this addition looks like a very useful tool."

And making the technique capable of very accurate anatomy might not be necessary, he adds. "Animators take the approach that, if it looks right, it's ok."

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The program analyses a character's body shape and works out the skeleton and muscles that fit inside (Image: Jian Zhang/Xiaosong Yang)